Capacitive sensors use the electrical property of “capacitance” to make measurements. Capacitance is a property that exists between any two conductive surfaces within some reasonable proximity. The capacitance is a measure of the amount of charge stored on each plate when a voltage is applied to one of the plates. The amount of charge that can be stored depends upon the distance between the plates, the surface area of the plates, and the permittivity of the non-conducting material between the plates, which is also known as the dielectric. The surface area of the plates is normally constant. Accordingly, with a capacitance-type sensor, if one of the two other factors is held constant, a change in capacitance correlates to a change in the non-constant factor. There are many applications for capacitance-type sensors. For example, if the area of the plates is constant and the dielectric is constant, but the position of the two plates relative to each other is variable, changes in capacitance correlate to changes in the distance between the plates, so a capacitance-type sensor can be used as a proximity sensor or a position sensor. A capacitance-type liquid level sensor typically comprises two conductive surfaces spaced a fixed distance apart from one another and oriented vertically within a storage vessel; when the liquid level changes, the permittivity of the dielectric between the plates changes and this changes the capacitance. That is, with a capacitance-type level sensor the surface area of the plates and the distance between the plates remains constant so that changes in capacitance are proportional to changes in the liquid level. Therefore, the capacitance between the two conductive surfaces of a capacitance-type level sensor increases as the level of the liquid rises and the permittivity of the dielectric changes. The maximum capacitance is measured when the conductive surfaces of the capacitive sensor is completely immersed in liquid.
When a capacitor is charged an electric field develops between the capacitor plates, developing a voltage difference therebetween. For a given capacitor there is a known relationship between charge, capacitance and voltage. The voltage is proportional to the amount of charge and the circuit detects an increase of capacitance when there is an increase in voltage. Because of the correlation between capacitance and voltage, the parameter measured by a capacitance-type sensor can be determined from the voltage measured at the capacitor. In this disclosure, by way of example, the apparatus and method are described in relation to capacitance-type level sensors, but persons skilled in the technology will understand that the same apparatus and method can be applied to other applications with other types of capacitance-type sensors to improve the accuracy of a measured parameter.
Accurately measuring the liquid level of a cryogenic liquid held in a storage vessel is a challenging application for sensors of all types. It is known to use capacitance-type level sensors for measuring cryogenic liquid levels inside a cryogenic storage vessel. However, with cryogenic liquids and storage vessels that are mobile, such as vehicular fuel tanks for storing liquefied natural gas, it can be especially challenging to accurately measure liquid level. Accurately detecting the level of liquid remaining for such applications is important because the consequence of an inaccurate level measurement can result in a vehicle being stranded if it runs out of fuel, or reduced operational efficiency if the vehicle is re-fuelled more frequently than necessary, that is, when a fuel tank is re-filled when there is still ample fuel remaining in the fuel tank. In addition, for vehicles that use a high pressure pump to deliver the fuel to the engine, there can be accelerated wear of the pump components if the pump is operated frequently when the fuel tank is empty.
The desired temperature for storing a liquefied gas depends upon the particular gas. For example, at atmospheric pressure, natural gas can be stored in liquefied form at a temperature of −160° C., and a lighter gas such as hydrogen can be stored at atmospheric pressure in liquefied form at a temperature of −253° C. As with any liquid, the boiling temperature for the liquefied gas can be raised by holding the liquefied gas at a higher pressure. The term “cryogenic temperature” is used herein to describe temperatures less than −100° C., at which a given gas can be stored in liquefied form at pressures less than 2 MPa (about 300 psig). To hold a liquefied gas at cryogenic temperatures, the storage vessel defines a thermally insulated cryogen space. Storage vessels for holding liquefied gases are known and a number of methods and associated apparatuses have been developed for removing liquefied gas from such storage vessels. The terms “cryogenic fluid” and “cryogenic liquid” are used herein to respectively describe a fluid or a liquid that is at a cryogenic temperature.
An additional challenge associated with measuring the level of cryogenic liquids as compared to other liquids, is that cryogenic liquids are typically stored near their boiling temperature, and there may not be as clear a delineation between the liquid and vapor spaces inside the vessel. Known capacitance-type level sensors for measuring cryogenic liquid levels, when operating normally, can be in error by as much as 20 to 25 percent.
Conventional systems need to periodically re-calibrate measurement circuits for capacitance-type sensors to prevent drifts in accuracy but it can be difficult to know when re-calibration is needed because capacitance-type sensors have a capacitance that is variable by nature, depending upon any changes in the parameter that the sensor measures. By way of example, drifts in accuracy can be caused by signal noise, manufacturing tolerances of circuit components that allow some variability in the performance of such components, the effect of temperature on component performance, and the effect of some components degrading in performance over time. Accordingly, for applications where a capacitance-type sensor is employed and the accuracy of the measured parameter is of particular importance, there is a need for more accurate and reliable measurements.